Gobsmacked

Definition:
stunned, surprised beyond all expectation (usually not happily)

Usage:
He gave me a bad review. I was gobsmacked!

Explanation:
The same reaction of shock and disbelief as if the subject person has just been slapped across the face.

Trivia:
Heard Mary Walsh, Newfoundland comic extrordinaire, say this one and loved it. Have found it useful ever since.